[x3d-public] review of various X3D statement implementationsfor X3DOM

[Joseph D Williams]

➢ A conceptual scene graph structure may look like this:

group1
  node 1: DEF "original"
group2
  node 2: USE "original"

then if you want to specifically identify between node 1 and node 2, don’t do that. In this, node 1 and node 2 are the same node with no way to distinguish between the two or method to make them different. Nor should there be. To be different, they must be not the same. 
Thanks, 
Joe 


Sent from Mail for Windows 10

From: Andreas Plesch
Sent: Wednesday, December 20, 2017 11:18 AM
To: Michalis Kamburelis
Cc: X3D Graphics public mailing list
Subject: Re: [x3d-public] review of various X3D statement implementationsfor X3DOM

Hi Michalis,

yes, that is how DEF/USE should be implemented. The point I am trying
to understand is if there is absolutely no distinction between the
original DEF node and a USE node which references the DEF node, how
can the USE node be identified, for example during picking ? The USE
node still needs to have at least a distinct ID, right ?

A conceptual scene graph structure may look like this:

group1
  node 1: DEF "original"
group2
  node 2: USE "original"

There are still two nodes. node 2 is not a copy of node 1 but is still
a separate node which happens to reference node 1.

Or

group1
 node 1: DEF "orginal"
group2
 node 1 (inserted a second time during scene graph construction)

Closer to spec. but makes it hard (impossible?) to distinguish the two
node 1 instances from each other.

So I was thinking something like an additional ID may have to exist:

1. group1
2.  node 1: DEF "orginal"
3. group2
4.  node 1 (inserted during scene graph construction)

?

-Andreas




On Wed, Dec 20, 2017 at 11:03 AM, Michalis Kamburelis
<michalis.kambi at gmail.com> wrote:
> 2017-12-20 15:51 GMT+01:00 Andreas Plesch <andreasplesch at gmail.com>:
>> (DEF/USE in x3dom: There are a several x3dom issues about picking a
>> USE Shape. Since x3dom literally places the referenced DEF Shape in
>> the scene graph, the picking actually reports the node of the
>> referenced DEF Shape when the USE Shape is picked. This is because
>> x3dom actually does not keep a distinct representation of the USE node
>> at all. Except for the fact that the DEF node now has two parents.
>> This may be too literal an interpretation of the DEF/USE concept, and
>> I wonder if in the spec. there is a section which explains how the
>> node which has a USE reference is still a distinct node. I suspect
>> that keeping such a distinct node around is how most x3d browsers work
>> ?
>
> This DEF / USE (in x3dom) treatment is required by the X3D
> specification, and it (should) be the same in all VRML / X3D browsers.
> I know it is the same in view3dscene and Castle Game Engine. When we
> encounter a "USE Xxx" then internally we *do not* make a copy of the
> "Xxx" node, instead we make an additional link to the "Xxx" node that
> in effect has 2 parents. This implementation is most natural and
> practically required by the X3D specification:
>
> http://www.web3d.org/documents/specifications/19775-1/V3.2/Part01/concepts.html#DEFL_USESemantics
>
> """
> The USE statement does not create a copy of the node. Instead, the
> same node is inserted into the scene graph a second time, resulting in
> the node having multiple parents (see 4.3.5 Transformation hierarchy,
> for restrictions on self-referential nodes).
> """
>
> And that's a very good concept, in my opinion: it allows to use DEF /
> USE mechanism for various optimizations. Since you have only one copy
> of the node, you should also be able to have only one copy of various
> internal things (like OpenGL VBO data) associated e.g. with Shape that
> is USEd multiple times. This is really powerful, you can construct a
> large scene by USEing the same shapes multiple times, and it will
> still render fast and not eat too much memory:)
>
> In contrast, expanding the PROTOtypes makes a copy on the contents.
>
> Regards,
> Michalis



-- 
Andreas Plesch
Waltham, MA 02453

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